Parts Required:

Coolant

Hot Tip:

Wear work clothes

Performance Gain:

Better engine cooling

Complementary Modification:

Replace thermostat

Changing and flushing your engine coolant is about a once-every-two-years maintenance task. It's a straightforward job in many ways, and with most cars it's about as involved as an oil change. With the A4, particularly with the 1.8T engine, the job is complicated just a bit, enough that it's not a simple drain-and-fill deal.

The first complication is that the easiest way to drain the radiator is by opening the drain petcock, which unfortunately is blocked by the side-mounted intercooler's ducting. The best way to get this out of the way is by removing the bumper. It isn't not a hard job (see the project that addresses this project), but it does add a little extra wrenching. If you decide to drain the radiator by disconnecting the lower radiator hose instead, you'll at least need to remove the plastic undertray (which needs to be removed in order to detach the bumper in either case). This in turn means you'll need to elevate the front end of the car (see the Jacking Up project).

The second complication is that the engine block (2.8 V-6) or water pump (1.8T) must also be drained. Again, not a big deal, but another step that must be completed. Third, a couple extra steps are necessary in order to ensure that air can escape from the top of the cooling system as coolant is added. This requires raising the coolant reservoir and exposing an air bleed hole in one of the two hoses that travels between the engine compartment and the heater core.

Figure 1

The coolant reservoir is hard to miss under the hood of the A4. Unlike some cars, the A4 does not have a cap on the top of the radiator; the cap of the coolant reservoir also keeps the system pressurized (in other words, treat it carefully, like a traditional radiator cap when the system is hot and under pressure to avoid burns from escaping steam).

The lower radiator hose enters the driver's side of the radiator, just underneath the front of the car and above the piping leading to the side-mounted intercooler. If you choose to disconnect the radiator hose here, insert a screwdriver underneath the metal clip securing the hose in place.

As far as draining the radiator goes, I think it's easier to use the radiator drain petcock on the front of the radiator (just on the other side of the radiator from where the lower radiator hose connects). The red screw that opens the petcock is easy to spot, but the edge of the intercooler ducting must be moved slightly to access it.

Place a drain pan underneath the car and loosen the petcock screw until you get a good flow of coolant coming out. The stream moves around a good bit as it flows down out of the outlet and along other things below it, so be ready to move the drain pan, and make sure you mop up any spilled coolant. The sweet coolant is attractive and poisonous to animals who might lap it up, or to children who might encounter it on the floor or in a container, so please be safe in handling it.

Continue draining the cooling system by removing the drain plug from the water pump. The plug head is 19mm in size and has a metal washer-type gasket that should be replaced. The camera was pointed straight up at the underside of the water pump for this photo.

This photo gives a clearer view of the drain plug. Once you've loosened it you can remove it by hand, but beware that a strong stream of coolant will soon be running down your arm, possibly under your sleeve and soaking your clothes. Don't ask me how I know. Have your drain pan handy, of course. After the coolant flow stops, tighten the drain plug to 19 ft/lbs.

It's a bad thing to have air trapped in an engine's coolant system. It can impede the flow of coolant and cause hot spots to form. Therefore it's important to make sure that air can escape as you fill the system. Remove the three Phillips screws holding the coolant reservoir in place so that you can elevate it about four inches and make the reservoir the high point in the system (see the oil change project for more detail on moving the coolant reservoir). Volkswagen/Audi has a fancy tool with which to do this; I've resorted to a high-tech cotton towel.

You must also burp air from the hoses that run to the heater core underneath the windshield cowl. You can reach the hoses by first removing the large plastic cowl covering that also hides the battery. The ribbed plastic piece covers the two hoses that lead to and from the heater core. Also note the tube that exits the battery just to the left of the hoses. You can easily move this hose out of the way to improve your access.

The ribbed plastic covering is split down the back side to allow for removal. You should be able to move it enough to access the hose clamp for the left (passenger side) hose. Mine was secured with a black zip tie that had to be cut first before I could move the covering. Squeeze the clamp with a pair of pliers to release it temporarily while you slide the hose out a couple inches. The white circular area on the top of the hose contains a small hole in the middle. By moving the hose a bit, you will open up that hole to allow air to escape during the coolant fill.

Here I've removed the heater hose all the way to show you that the hole is in the hose, not in the hard pipe to which the hose connects. This means you only have to slide the hose down until air can pass through this hole.

Rather than just filling up the cooling system with radiator fluid and calling it a day, it's a good idea to flush it with water first. This means you have to drain the system twice, but it ensures a more complete coolant flush. Use a garden hose to fill the system with regular water. When water flows out of the small hole in the heater hose, you know it's full. Put the cap on the coolant reservoir and slide the heater hose back into place. If you've supported the reservoir with a towel as I have, be sure the towel isn't near any moving parts--or simply put the reservoir back into position during this warm-up procedure. Start the car and let it come up to temperature. Turn the heater controls inside the car to the full-hot position to ensure that the heater core gets flushed. Then, once the car has cooled down again, remove the cap on the reservoir. Drain the radiator and water pump again. Expose the hole in the heater hose again and fill the system with a 50-50 mix of coolant and distilled water (or pre-mixed coolant). Use the same kind of coolant you found in the system - if it's green, stick with green; if red, refill with the red coolant. You don't want to mix these two kinds of coolant, as this can lead to engine damage. Once the system is full, slide the heater hose back into place and return the coolant reservoir to its normal position and secure it with the three Phillips screws. Top off the coolant reservoir to the MAX mark on the side of the tank. Start the engine and let it come up to temperature. If the lower radiator hose is hot to the touch, then the engine is up to the proper temp. Check the coolant level in the reservoir; if it's not at the MAX mark, wait for the engine to cool down and top off the fluid.

If you removed the tubing from the battery to gain better access to the heater hoses, you'll need to replace it before you button up the cowl cover. The tubing may have been dislodged at the bottom end during your manipulations of the heater hose. The bottom end inserts into a hole in the rubber grommet through which the heater hoses pass. I've removed the ribbed plastic covering on the heater hoses in order to show where this drain tubing goes. This view is looking straight down from above, with the camera lens parallel to the top of the battery. Now, replace the bumper if you removed it, as well as the undertray. Lower the car to the ground to finish the job.

Comments: I have a 2004 audi a4 2.0 . the car overheated and has been fixed but now I'm sitting with the problem, when pulling away in 1st gear it has power up until 3000rpm and then nothing. its like this in all the gears. we have had the head pressure tested and skimmed, intake manifold replaced, a bunch of sensors replaced, coils replaced, spark plugs replaced, fan replaced, throttle body & throttle pedal tested and yet not power. what could be causing this?

April 7, 2015

Followup from the Pelican Staff: I would check exhaust backpressure also perform a fuel pressure and volume test. - Nick at Pelican Parts

Amir

Comments: have an Audi 4 2 ltr 2007. there is an alarm light that is on showing like the radiator cap is missing or open sprinkel like. the coolant level is OK and the car does not get hot. any idea what went wrong?

March 25, 2015

Followup from the Pelican Staff: That is washer fluid, check your owner's manual to confirm this. Then fill it to shut the light off. - Nick at Pelican Parts

antoin

Comments: The image of my previous post didn't come through. I will try once again...

In my case, for an Audi A4 1.8T quattro avant 1998, it was not necessary to remove the bumper to get to the radiator drain petcock.

It is sufficient to remove the plastic grill and attach a piece of hose 30cm where coolant emerges as you mention in Figure 7. If the hose is flexible enough it will fit tight just by pushing it.

See image.

March 15, 2015

Followup from the Pelican Staff: Thanks for sharing your installation process and experience. These type of comments add so much to the Pelican tech community.
- Nick at Pelican Parts

Will

Comments: Can you advise how many litres of coolant are in a 2001 PASSAT 1.8T AWT Engine.

March 9, 2015

Followup from the Pelican Staff: I don't have that info handy. Give our parts specialists a call at 1-888-280-7799 and they can help figure it out. - Nick at Pelican Parts

H.R.

Comments: I recently changed the coolant bottle in my 2001 Passat AWM. Now there is no heat and the coolant is boiling up to the coolant tank and exiting through the overflow. By the time it cools off the coolant tank is empty. I was getting heat before, only changed tank due to cracks. The warning light comes on but the car does not overheat it stays right at 190 deg. How do I fix this? Thanks in advance

February 17, 2015

Followup from the Pelican Staff: Sounds like air is trapped in the cooling system. Try bleeding the cooling system. Once you think the air is out. Warm engine up, then shut it down. Once cool, recheck cold coolant level. - Nick at Pelican Parts

rljones

Comments: Question I was changing the thermostat on a 2000 golf 1.8 turbo.does the thermostat go in first or the boringgasketmy friend took it out and didnt pay attention.

January 22, 2015

Followup from the Pelican Staff: The gasket (O-ring) has to make contact with the housing, it should go in after the thermostat. - Nick at Pelican Parts

GARRY

Comments: NOTE OF CAUTION ; I HAVE A 1.8T 20 V NEW BEETLE ....I HAVE DONE SEVERAL MODS TO HELP WITH ENGINE COOLING ....I READ THE INFO ON WATER WETTER TYPE COOLANTS TO HELP DECREASE WATER TEMPS . i ADDED A BOTTLE OF WATER WETTER TO MY RESERVOIR . I HAVE NEVER HAD AN OVERHEATING PROBLEM IN 5 YRS WITH THIS CAR ....ABOUT 10 MILES AFTER PUTTING IN WATER WETTER , MY ENGINE OVERHEATED AND BLEW 2 O RINGS AND DO NOT KNOW WHAT ELSE YET ....MY WATER PUMP SEEMS FINE AND THERMOSTAT OPENS . I FOUND THAT MY RESERVOIR RETURN LINE WAS COMPLETELY PLUGGED AT THE RESTRICTOR ...I HAVE READ THAT WATER WETTER CAN BE PROBLEMATIC AFTER SEVERAL CYCLES , TURNING INTO A STICKY GOOPY SUBSTANCE THAT IS DIFFICULT TO REMOVE FROM SYSTEM ...I DO NOT KNOW FOR CERTAIN WHAT CAUSED THIS OVERHEAT BUT IT SURE IS A COINCIDENCE MY ONLY OVERHEAT IN 5 YRS ON A COLD DAY AFTER DRIVING 6 MILES WOULD DO THIS , AFTER ADDING WATER WETTER TO COOLANT ....

January 14, 2015

Followup from the Pelican Staff: I would check if there isn't an underlying problem.

Thanks for the additional info. We appreciate it.
- Nick at Pelican Parts

darrell

Comments: I have no water at all coming out of my heater hose, bleed point. removed hose completely and still not a drop of water. could it have been bypassed somewhere.
?

January 14, 2015

Followup from the Pelican Staff: It could have been. What vehicle are you working on? - Nick at Pelican Parts

Deborah

Comments: Will it damage my car if I temporarily ues water in the radiator instead of the special fluid? I cant get the car serviced for 3 weeks

January 8, 2015

Followup from the Pelican Staff: As long as the cooling system doesn't freeze. If it freezes, the engine will be damaged. - Nick at Pelican Parts

Oscar

Comments: Great and very helpful walk-through. I have a question, I plan on flushing my 2001 Audi A4 1.8T as I am going to update the G12+ to the G13 coolant. Is radiator flushing fluid recommended? Or would just using plain water suffice. Thank you!

January 7, 2015

Followup from the Pelican Staff: Plain water is fine. - Nick at Pelican Parts

GARRY

Comments: I HAVE A 1.8T TRANSVERSE NEW BEETLE . MY ROUND RESERVOIR IS
ALREADY THE HIGHEST POINT IN SYSTEM I THINK, AND IF SO IS IT NESSESCARY TO ELE3VATE IT MORE ? ALSO ON THE NEW BEETLE I DO NOT KNOW WHERE THE HEATER HOSE IS , AND FROM PICTURE HARD TO TELL WHERE THE WATER PUMP DRAIN PLUG IS ? ON THE ROUND RESERVOIR I CAN SEE NO FLUID MARK FOR LEVEL ? HELP...

January 7, 2015

Followup from the Pelican Staff: You should not have to elevate the reservoir to fill and bleed the system. - Nick at Pelican Parts

A89

Comments: Hello, I have a 2004 Audi A4 1.8T, my coolant light came on and I added coolant to the appropriate line , and my coolant light still shows up any ideas what else it can be ??

January 3, 2015

Followup from the Pelican Staff: The coolant level sensor could be stuck. Check if that is the case. - Nick at Pelican Parts

Joe

Comments: Hi my name is Joseph and I have a 2002 vw passat v6 and my heat our ac doesn't work. There aren't many people here who work on vw's but the one guy I took it to said that who ever had the car before me had used green coolant, which caused the coils to get cloged up. He also said I had to replace the whole thing which would cost 1000 at min. Should I try flushing first?

January 1, 2015

Followup from the Pelican Staff: Can you confirm there is no flow through the heater core? I would try and run water through it and see if it has good flow before replacing it. - Nick at Pelican Parts

Williex

Comments: I have also tried to use compressed air in the return hose to see if it will bubble in the coolant tank . But I don't see any.

December 13, 2014

Followup from the Pelican Staff: Ok, got it. - Nick at Pelican Parts

williex

Comments: I have a 2002 Volkswagen Beetle 2.0 liter. Have replaced water pump and thermastat and housing. I was overheating and didn't have heat. I'm still overheating.and know heat . Flushed heater core and it was completely cloged but was able to get flow not sure if was clear . Have Luke warm heat now on full blast but still overheating. Now I have found that the retun hose to the coolant tank has no flow. I have tried prestone flush and a hanger down the hose threw the metal nipple on the head and still no flow and still overheating.is there an air pocket? And to get the flush out do I just open the cock pit on rad and put water hose in resivior start car and let it run till clea water is coming out of radiator. Help

December 13, 2014

Followup from the Pelican Staff: There should be coolant flow out of the bleeder hose as you fill it with the engine cold. If plugged, air may remain trapped. - Nick at Pelican Parts

Tom

Comments: The plastic bleeder screw on the coolant pipe leaks on an A4 1.8T, can it simply be replaced with a metal one?

November 22, 2014

Followup from the Pelican Staff: I don't think so. You may need a new pip.

- Nick at Pelican Parts

Brian

Comments: How do you mix it up you first fill distilled water in the car then fill the G12? My G12 comes in litres so I got 3 litre here

October 31, 2014

Followup from the Pelican Staff: Mix the two first. 50/50, then add to vehicle. - Nick at Pelican Parts

Phil

Comments: My daughter flushed and re-filled her radiator on her 2004 Passat. Now she does not have heat from the heater. Can you assist?

October 19, 2014

Followup from the Pelican Staff: Sounds like air is trapped. Try bleeding the cooling system. - Nick at Pelican Parts

aj

Comments: excelent detail, not every one handy to do it though

September 20, 2014

Followup from the Pelican Staff: Thanks for the feedback. - Nick at Pelican Parts

pat

Comments: my friend also has a Audi A4 cabriolet'07 and has been filling the coolant compartment with windshield washer fluid by mistake, what should she do?? please answer

September 10, 2014

Followup from the Pelican Staff: Flush the entire cooling system. Then replace the coolant with new. - Nick at Pelican Parts

Memo

Comments: Is the radiator hose supposed to just come straight out after lifting the clip away? Or does it need some turning? I can't detach it from the radiator and I don't want to break or damage it by pulling too hard since its plastic. Thanks.

August 20, 2014

Followup from the Pelican Staff: The can get stuck, from being on the neck so long. Try wiggling it back and forth while you pull. It also wouldn't hurt to clean any dirt out and spray soapy water into the neck / hose are. - Nick at Pelican Parts

CassieB

Comments: Hi Nick
Thanks for your help. I would love to phone your parts dept but unfortunately I reside in South Africa, thus the reason for the part nr. If you can perhaps help with that I would appreciate it.
Thank you again

August 11, 2014

Followup from the Pelican Staff: I can't be much more help than my original answer.. - Nick at Pelican Parts

CassieB

Comments: Hi There I need some info. I have a 1998 A 4 Audi. Underneath the manifold there are 2 metal pipes. The lower one comes out of the waterpump. That one needs replacing and I do not heave a part number or name for the part. Would you be so kind as to let me know what it might be

August 10, 2014

Followup from the Pelican Staff: This is a coolant pipe, no fancy name. Give our parts specialists a call at 1-888-280-7799 and they can help figure out which part or repair kit you need.
- Nick at Pelican Parts

Bizobby

Comments: I have a 2005 Vw passat... Coolant seems to be leaking from the bottom of the reservoir... From a small port there... Should it be a hose connected to that port?

July 31, 2014

Followup from the Pelican Staff: This could be the overflow drain. If the vehicle overheats or loses coolant through the cap, it will come out of this drain. I would check if coolant is leaking past the cap. - Nick at Pelican Parts

Robert

Comments: I have a 2005 Audi A4 1.8T. A few months backmy coolant light came on. After some investigating I found a leak from the plastic bleeder on the coolant pipe just under the engine cover. I replaced this with a metal one and the leak stopped. I topped off the coolant level and the light went off but then came back on after driving a few seconds. I rechecked the coolant level and it is full. Could this be due to air in the system? Also after reading some posts on Audi forums, people have stated that the coolant should be pink. My coolant is a brown color, would this cause the light to come on? Should I drain the system and replace with fresh coolant following your instructions? Thank you in advance.
Robert

July 28, 2014

Followup from the Pelican Staff: COlor will not cause the light to come one. You may have low coolant, air trapped or a dirty sensor. If the sensor is dirty, it could stick and give an incorrect coolant level reading. - Nick at Pelican Parts

Dingo

Comments: Fantastic Instructions and supporting images! My problem - I'm having troubles with the heating and I understand it could be an air pocket trapped in the heating coil. I drained everything as per instruction, but the heater still stays luke warm and only reaches that point when the engine is hot... I assume it's just pumping in the heat from the engine bay. I can hear the 'heater' kick in once I turn it on. Could it be the heater coil, thermostat, or air pocket? Cheers again!

July 21, 2014

Followup from the Pelican Staff: Sounds like air is trapped to me. Try bleeding the system again. - Nick at Pelican Parts

AthensDion

Comments: Great, great information. Thank you very much.

July 21, 2014

Followup from the Pelican Staff: Thanks for the feedback. Glad we could help.
- Nick at Pelican Parts

shad

Comments: I have an 01 passat 1.8t - I recently had the head gasket, water pump, thermostat, rear coolant flange and radiator replaced. But it still seems as if its not circulating. Upper hose is hot but no flow. Temp gauge stays at 190 while running but when you shut it of makes a gurgling sound. Could this be a air bubble problem?

July 9, 2014

Followup from the Pelican Staff: If the system is not losing coolant, you may have air trapped. Try bleeding the cooling system again. - Nick at Pelican Parts

Diggingholes

Comments: What is the process, specifically of "bleeding" the system?
I understand I could just empty the whole thing, but I really don't want to do that, unless that is what you mean by bleeding. Thanks!

Comments: I recently replaced my coolant reservoir, but didn't follow proper protocol regarding the bleed hole near the heater core and raising the reservoir. Consequently, my car's temperature gauge started acting funny not going to normal position, and going back to zero etc and the car has been running very rich enough to throw CEL once and having trouble with cold starts. Could this all be related to air bubbles in the system? How do I go about removing the air bubbles, without having to drain all the coolant? Thanks.

2001.5 Audi A4Q AWM Sedan

July 2, 2014

Followup from the Pelican Staff: If the coolant sensor is not submerged in coolant, it could throw an incorrect reading to the DME. I would bleed the cooling system and see if the engine begins to run normally. - Nick at Pelican Parts

hector

Comments: i can't find the drain plug on the water pump. can you please tell me how to find it?

June 14, 2014

Followup from the Pelican Staff: It is visible from below. See attached photo. If your water pump has been replaced previously, it may not have one. - Nick at Pelican Parts

Marie

Comments: How do I check if the thermostat is stuck open? Or am I better off just taking it to yet another mechanic?

May 20, 2014

Followup from the Pelican Staff: Monitor hose temperature. The hose at the thermostat will remain cool until the thermostat opens. If it slowly warms up as engine runs, the thermostat may be stuck open. - Nick at Pelican Parts

Marie

Comments: Also I haven't had heat in the car for quite some time. The temp gauge on the dash never moves.

May 16, 2014

Followup from the Pelican Staff: I would start by checking your coolant level. Then check to see if the thermostat is stuck open. The engine should warm up before coolant begins to flow through the thermostat. - Nick at Pelican Parts

Marie

Comments: I have a 1998 Audi A4. When I put coolant in, it just pours out the bottom of the tank. Is there supposed to be a plug to prevent this from happening?

May 15, 2014

Followup from the Pelican Staff: There is a level sensor in the tank that could be leaking, or a crack in the tank. - Nick at Pelican Parts

Alex

Comments: Thanks for this. My 1998 1.8 Passat is missing the lower part of the bumper and the undertray, so I am just going to open the petcock and refill a few times then use a radiator cleaner product. MY question is, what size hose do I need to put over the petcock valve/spigot? I would rather buy an exact fit hose to keep the mess to a bare minium. Thanks.

April 25, 2014

Followup from the Pelican Staff: I would buy a clear hose with an ID of 10mm. - Nick at Pelican Parts

Fred

Comments: Great write up Nick! the best I've seen : One question, is it OK to skip the draining of the water pump step? I would think it would get cleaned out in the flush w/water step. Thanks!!

March 31, 2014

Followup from the Pelican Staff: You can, just be Ok with some old coolant staying in the engine. No guarantee it will come out with the flushing. - Nick at Pelican Parts

Fred

Comments: Can't you just get to that petcock drain with a long screwdriver after popping out the little grill in the lower bumper without removing the bumper as suggested?

March 27, 2014

Followup from the Pelican Staff: Yes, I have done it that way before. - Nick at Pelican Parts

Mr Finny

Comments: Excellent. Love the pictures. Just cleared the drain holes below the battery and found the battery vent hose pinched off, under the battery. In checking your pictures, I now see where the vent hose is supposed to go. Perfect. I hate to have to take the bumper off, however, it has some damage and needs to be fixed anyway so, might as well drain the rad. Thanks again.

October 27, 2013

Followup from the Pelican Staff: Thanks for the feedback. Glad we could help. - Nick at Pelican Parts

Steve

Comments: Thankyou. This info is just what I needed to properly bleed the air out of a friends car we had to replace a hose on. Great photo's and instructions.

September 29, 2013

Followup from the Pelican Staff: Thanks for the feedback. Glad we could help.
- Nick at Pelican Parts

johnboy

Comments: thanks your explanation is great now i know what to do

August 1, 2013

Followup from the Pelican Staff: Thanks for the feedback. Glad we could help. - Nick at Pelican Parts